1. Field Of The Invention
The present invention relates to apparatus for calibrating color imaging apparatus. More particularly, the invention relates to spectrometer apparatus for self-calibrating a color image scanner.
2. Description Of The Prior Art
Color image scanning is a process of converting an image, recorded on either a transparency or a reflective print, to an electronic image. Scanning, as such, is commonly employed as a first step in the transmission of an image from one storage medium to another, or in the enhancement or the analysis of the image prior to its transmission.
With an image-bearing sheet held in a given plane, a color image scanner measures the optical density of the image by illuminating the sheet with polychromatic light. Commonly, a color image scanner measures the amount of light in a given color space, e.g. red (R), green (G), and blue (B), transmitted through, or reflected from, the image-bearing sheet. In doing so, the scanner effectively divides the image into discrete picture elements, or pixels, and assigns to each a number or value representing an average density for each color measured. Commonly, the pixels are arranged in rows and columns to form a two-dimensional grid with the density of each pixel corresponding to a relatively small portion of the overall image.
As is appreciated by those skilled in the image scanner art, calibrating the spectral sensitivity of a color image scanner is important to provide accurate reproduction of all colors in the image.
The spectral sensitivity of a color image scanner is determined by the spectral characteristics of a variety of scanner components such as the spectral content and sensitivity of its light source, the spectral transmissivity characteristics of filters, lenses and the like, and the spectral sensitivity of its image sensor. All of these factors, however, are likely to change, component to component, particularly with the use of a scanner over long periods of time. Furthermore, even with complete component stability, an original component may be replaced from time to time with a new component having a somewhat different characteristic.
One commonly employed technique for performing a calibration operation on a color film scanner is to employ a reference photographic film containing a set of standard color patches and to measure the scanner response to each patch. To recalibrate the scanner, each color patch is scanned again and its measured value compared with the corresponding previously recorded response.
This technique suffers from a disadvantage in that a film having standard color patches is expensive. This is particularly a problem for a scanner useable with a variety of film types because a reference film of each type would be required. Furthermore, if a film having standard color patches is not stored under conditions required for stability, an apparent change in scanner calibration may occur that is actually due to a change in film.